1. Field of the Invention
The present invention relates to a laser beam optical system which includes a laser beam light source and a laser beam scanning optical system for use in reading and writing images.
2. Description of the Related Art
Conventional laser light sources of the optical disk pick-up and laser printer image-writing head which uses a Fresnel lens as a collimator lens are well known.
Japanese Laid-Open Patent Application No. 3-56901, for example, discloses a collimator lens system construction having a Fresnel lens configuration capable of correcting variations in refractive index (typically, the refractive index is a function of the wavelength) produced when the laser diode oscillation wavelength fluctuates. However, in the aforesaid collimator lens system, consideration is given neither to the variations in focal length arising from shape variations caused by expansion of the Fresnel lens material in conjunction with the heat generated by the laser diode, nor variations in the focal length arising from variations in the refractive index of the lens material itself caused by temperature fluctuations. A Fresnel lens necessarily must be made of resin material for ease of manufacture, however, correction of variations in focal length in accordance with changes of shape and refractive index caused by temperature fluctuation are unavoidable.
Japanese Laid-Open Patent Application No. 1-206682 discloses an arrangement which cancels out the variations in focal length of the Fresnel lens when wavelength fluctuations occur due to the heat generated by the laser diode with changes in focal length based on changes in the grating spacing due to heat expansion of the lens material. Since the lens surfaces other than the Fresnel lens are flat in this lens system, said lens materials are limited to materials having a large thermal expansion coefficient when used to correct the focal lens by changing the grating spacing of the Fresnel lens because such focal lens correction can only be accomplished via the Fresnel lens. However, this arrangement does not consider changes in focal length based on variations in refractive index of the lens material induced by temperature changes.
Laser beam scanning optical systems used as the image-writing head of a laser printer provide an arrangement of a pair of anamorphic lenses in disposed anteriorly and posteriorly to a deflecting device to correct surface tilt of the reflective surfaces of the deflecting device (polygonal mirror) (refer to U.S. Pat. No. 3,750,189). Heretofore, these anamorphic lenses have been made of glass, but in recent years such anamorphic lenses made of polycarbonate resins and the like have been developed. However, resin lenses provide weak power characteristics as the refractive index is reduced with increasing temperature, and the focal length changes in the positive direction (lengthening direction).
On the other hand, an anamorphic lens construction via axial symmetry of the grating of Fresnel lenses having a diffraction effect has been disclosed in Japanese Laid-Open Patent Application No. 2-168220. When such an anamorphic Fresnel lens is used to correct surface tilt of the deflecting device, the focal length changes in the negative direction (shortening direction) due to enlargement of the grating spacing accompanying a temperature rise. In any case, if the lens focal point and length change, the beam spot moves on the image forming surface (scan line), thereby reducing image quality.